1. Field of the Invention
This invention relates generally to antennas used for space applications and more particularly to a hybrid parabolic reflector phased array antenna which is stowed in a collapsed state for launch and thereafter deployed to form a relatively large reflector type antenna when in orbit.
2. Description of Related Art
Extremely large scanning antennas for space applications and having limited scan requirements are well known. As the antenna is moved away from the earth, the scan angles are reduced, while the size of the antenna increases. The problem of deploying and steering very large antennas is formidable. Phased arrays generally have too many elements to be cost effective while reflector antennas have configuration problems in amount of blockage and performance degradation at the edges of scan.
Currently, large scanning antennas use parabolic reflectors with clusters of elements at and near the focal point to scan the beam. In order to steer the antenna, a large group of elements are used to transmit and receive. On transmit, phase-only control is preferred, while on receive both phase and amplitude controls are used. Moreover, on transmit, amplitude is uniform while in receive it is normally tapered. In order to distribute the power among many elements to reduce the heat concentration, the feed array is typically displaced forward of the focal point; however, this increases the size of the feed rapidly, with commensurate increase in blockage loss.
Apertures comprised of a plurality of reflector super elements, all having feed array generating respective antenna patterns, steer a composite beam pattern near the desired direction. In such apparatus, phase or time delay between elements is then used to fine steer the antenna. With large spacing between elements, however, grating lobes are formed, which is the classic problem of using a large element in a phased array. At beam positions between element pointing positions, there can be major grating lobes that sap the power from the main beam and that, in turn, raise serious clutter problems.
The present invention is directed to a hybrid parabolic reflector phased array antenna system which is stowable in a rocket and is deployable in space. The antenna includes a large torus which acts as a support structure for a plurality of small reflector cells called super elements, each including its own reflector and an array of feed elements. The torus supports a stretched reflector mesh and matching back-up catanary wires that provide a mechanism for pulling the reflector surface of the cells down to an exact paraboloid. A set of rigid corner posts for stretching the mesh fabric for forming multiple reflectors is also provided. The torus is also used to support individual super element feed arrays for each reflector. The super elements incrementally scan the beam by group selection of feed elements in each feed array with time delay phase control being used to steer the array factor so as to achieve fine steering. Each of the super elements scans incrementally with a selected group of feed elements varying between three and twelve, which are varied in position relative to the focal axis of the feed array. At intermediate positions, where grating lobes appear, the groups of feed elements are reduced in number and selected so as to steer precisely to this position, thus relieving the grating lobe problem. Other methods of mitigating the grating lobe problem include randomly selecting groups of elements about the optimum position, gradually shifting the selected group of elements from one position to another, randomly positioning the feed arrays about their respective focal points, and overlapping feed distributions to gradually shift the feed center and thus precisely adjusting the feed element pattern to agree with the array factor peak position.